This invention relates to the logging of magnetic properties of formations traversed by a borehole and, more particularly, to the logging of properties related to the direction of the earth's magnetic field at the time of deposition of each of the individual formations rather than to the nature of the materials comprising the formations.
The total magnetic field of an earth formation includes, firstly, a component induced by the earth's present magnetic field and, secondly, a remanent magnetic field. This second component, a remanent magnetism, is related to the direction of the earth's magnetic field at the time of deposition of the sedimentary rocks of the formation. Consequently, reversals of this remanent magnetism between formations indicate paleomagnetic reversals in the direction of the earth's magnetic field between the times of deposition of the formations. A borehole logging tool for identifying such paleomagnetic reversals will be useful in delineating between the sedimentary rock deposits of the formation and in age dating such formation deposits.